1. Field of the Invention
The present invention relates to an image reading apparatus capable of adjusting reflection light irradiated from a backside of an original document, and also relates to an image forming apparatus including an image reading apparatus.
2. Description of Related Art
A related art image reading apparatus such as a scanner, a photocopier, a facsimile or a multifunctional device (capable of serving at least two of the following functions, scanner, photocopier and facsimile) includes an image reading unit having a light source irradiating an original document to be read placed on an original document setting tray with light and a photoelectric conversion element photoelectrically converting the light reflected from the original document. Such a related art image reading apparatus uses the image reading unit to scan, while moving along, the original document, thereby reading the image of the original document (see, e.g., Japanese Un-examined Patent Application Publication No. 2001-169080). The related art image reading apparatus also includes a pressure plate pressing the original document placed on the original document setting tray. The pressure plate is white so as to serve as a back ground of an original document when the original document is placed on the original document setting tray and to detect whether or not the original document is placed the original document setting tray.
However, in such a related art image reading apparatus, when an image on a front side of an original document having images printed both sides is read, the light from the light source permeates through the original document, and an image on a backside of the original document becomes visible due to the light reflected from the white pressure plate. In this regard, the image on the backside of the original document is incorporated in the light reflected from the front side of the original document, that is, a phenomenon so called “show-through” is generated, and an image quality is significantly deteriorated. Moreover, in a related art image reading apparatus capable of reading the image on the front side of the original document by automatically feeding the original document, when the original document placed on the original document setting tray is read, there raises a problem that the image quality is deteriorated by, for example, a phenomenon of darkening an area of the image, etc. due to a different refection rate of a portion of the original document pressure plate pressing the original document. Consequently, the related art image reading apparatus generates such phenomena due to the reflection light irradiating the original document from the backside by reflecting the light permeated through the original document in a course of reading the original document.